The invention relates to an electrode for electrolysis processes, in particular to a cathode suitable for hydrogen evolution in an industrial electrolysis process. The electrolysis of alkali brines for the simultaneous production of chlorine and alkali and the processes of electrochemical production of hypochlorites and chlorates are the most typical examples of industrial electrolytic applications with hydrogen cathodic evolution, but the electrode is not limited to any particular use.
In the electrolysis process industry, competitiveness depends on several factors, the most important of which is the reduction of energy consumption, directly associated with the operating voltage. This justifies the many efforts aimed at reducing the various components of the latter, among which cathodic overvoltage must be counted. Even though cathodic overvoltages naturally obtainable with electrodes of chemically resistant materials (for instance carbon steel or nickel) not provided with catalytic activity have been considered acceptable in several industrial applications, the energy cost increase has by now made necessary the use of catalytic systems to facilitate cathodic hydrogen evolution. A solution practised in the art to depress cathodic hydrogen evolution overvoltage in electrolytic processes is given by the use of nickel substrates provided with catalytic coatings based on noble metals, such as platinum or ruthenium. This implies, however, a sensible cost increase due to the high price of noble metals. Some catalytic systems based on materials alternative to noble metals were thus implemented, among which various nickel alloys with other metals such as molybdenum, manganese, titanium, tungsten, vanadium, indium, chromium and zinc, obtained by galvanic co-deposition or by thermal processes, such as flame or plasma thermal spraying. Of all these systems, nickel-molybdenum galvanic alloys have given the best results, their catalytic activity being nevertheless still far from that obtainable by means of noble metal-based coatings. The cited alloys do not show, in fact, any true synergistic effect, and their increased activity towards hydrogen evolution reaction is due to a purely geometric effect, as demonstrated by the fact that the Tafel slope they exhibit is equivalent to that relevant to pure nickel (−120 mV per current decade).
The metal alloys of nickel and molybdenum also present the drawback of an insufficient open circuit stability in alkaline environment, which accounts for the unsuitable duration of the corresponding cathodes used in chlor-alkali processes or in water chlorination by hypochlorite generation. The problem is further enhanced in cases where such electrodes are subjected to anodic polarisation, be it accidental (for instance for a malfunctioning of the relevant electrolyser) or programmed (such as the case of chlorinating cells, wherein the potential inversion is used for the periodic cleaning of electrodes from scaling deposits).
It is, therefore, necessary to identify a noble metal-free catalytic formulation for an electrode suitable for operating as a hydrogen-evolving cathode in industrial electrolysis cells presenting characteristics of improved hydrogen evolution cathodic potential and of higher stability in alkaline environments, optionally also at open circuit or under anodic polarisation.